The present invention relates to a seat sensor disposed inside the seat of a vehicle for detecting the existence of a passenger and to a detection device using the seat sensor.
Recently, as vehicles have become equipped with many kinds of high-level functions, various seat sensors have been used to control functions such as actuation of an air bag system or indication of a seatbelt sign by detecting a seated passenger and determining a weight of the passenger.
A conventional seat sensor is described with reference to FIG. 11 through FIG. 15.
FIG. 11 is a cross sectional view of the conventional seat sensor. In FIG. 11, upper resistor layer 2, which is a pressure sensitive resistor element and is made of resins with conductive particles and insulating particles dispersed therein, is formed by printing underneath top sheet 1 made of flexible insulating films like polyethylene terephthalate or polyimide.
Bottom sheet 3 made of a flexible insulating film is disposed under top sheet 1. A pair of comb-like conductive layers 4 are formed over bottom sheet 3. Conductive layer 4 is formed by etching a copper foil bonded to a sheet or by printing flexible resins like polyester or epoxy resin, including silver or carbon dispersed therein, over a sheet. Lower resistor layer 5, which is a pressure sensitive resistor element and is made of resins with conductive particles and insulating particles dispersed therein, is formed by printing over conductive layer 4 so that bottom resistor layer 5 is opposed to upper resistor layer 2 formed underneath top sheet 1.
Insulating spacer 6 is made of insulating film 6A and adhesive 6B and 6C coated on both surfaces of insulating film 6A. Opening 6D is formed in the center of insulating spacer 6. Top sheet 1 is bonded to bottom sheet 3 via insulating spacer 6, and opening 6D keeps a predetermined distance between upper resistor layer 2 and lower bottom resistor layer 5 which oppose each other. The conventional seat sensor is produced in this manner.
As shown in an enlarged cross sectional view in FIG. 12, upper resistor layer 2 and lower resistor layer 5 are made of insulating resins 2A and 5A, including epoxy resin, phenol resin polyester resin or the like. The resistor layers include conductive small-diameter particles 2B and 5B, such as carbon black particles, and insulating particles 2C and 5C, which have large diameters as compared with the thickness of insulating resin layers 2A and 5A, and these particles are dispersed in insulating resins 2A and 5A.
The seat sensor formed as described above is placed inside the seat of a vehicle. When a passenger sits, top sheet 1 deflects due to the weight of the passenger and top resistor layer 2 makes contact with bottom resistor layer 5 as shown in a cross sectional view in FIG. 13. In a first stage of contact between upper resistor layer 2 and lower resistor layer 5, insulating particles 2C and 5C (which partially protrude from the surfaces of insulating resin 2A and 5A) are in contact with insulating layer 5A and 2A, respectively, as shown in an enlarged cross sectional view in FIG. 14A.
Then, as further load is applied to the seat sensor by a passenger, insulating resin 5A including conductive particles 5B dispersed therein makes contact with insulating resin 2A including conductive particles 2B dispersed therein. A contact area between insulating resins 2A and 5A increases as the load increases. Therefore, resistance between the two conductive layers 4 decreases. When a passenger is seated completely and the load applied to the seat sensor reaches a constant value, the contact area between upper resistor layer 2 and lower resistor layer 5 becomes a constant value. Therefore, resistance between the two conductive layers reaches a constant low value not more than a predetermined value as compared with the resistance in a first stage of contact.
Resistance of the seat sensor is a summed value of the resistance at each point where upper resistor layer 2 makes contact with lower resistor layer 5. A vehicle electronic circuit coupled to conductive layer 4 detects resistance of the seat sensor and determines whether the seat has a passenger, and whether the weight of the seated passenger is not less than a predetermined value. The electronic circuit is constructed so that it can indicate to a seatbelt usage sign or a control air-bag system whether the weight of the seated passenger is not less than a predetermined value.
However, in the conventional seat sensor described above, when a load is repeatedly applied to the seat sensor, particles 2C and 5C peel away with insulating resins 2A and 5A at an interface, and a gap 7 is formed between them. Because not only insulating particles 2C and 5C but also conductive particles 2B and 5B are dispersed in upper resistor layers 2 and 5, respectively, the holding strength of insulating resins 2A and 5A for insulating particles 2C and 5C is low.
Moreover, insulating particles 5C bite into conductive layer 4 by repeated loading and the amount of insulating particles 5C protruding from the surface of insulating resin 5A decreases because low resistor layer S is formed over conductive layer 4. As a result, a contact area between upper resistor layer 2 and lower resistor layer 5 changes and becomes different from the contact area at the beginning of use, and resistance reduces inversely with the applied load. In other words, a problem occurs in that resistance properties become unstable.
The present invention addresses the problem, and it provides a seat sensor having stable resistance properties and a detection device using the seat sensor.
A seat sensor of the present invention comprises a top sheet made of an insulating film, and an upper insulating layer which has insulating particles partially protruding from a surface of the upper insulating layer and which is formed underneath the top sheet. The seat sensor further comprises an upper resistor layer formed on the upper insulting layer, a bottom sheet made of insulating film disposed under the top sheet, a lower insulating layer which has insulating particles partially protruding from a surface of the lower insulating layer and which is formed on the bottom sheet, a lower resistor layer formed on the lower insulating layer, and an insulating spacer layer formed between the top seat and the bottom sheet.
The seat sensor of the present invention has a conductive layer between the insulating layer and the resistor layer, if necessary. The seat sensor also has an insulating spacer layer having ends that smoothly slope.